set_skin doesnt work on player (only on custom NPC)

[amytimed]

im doing properties.set_skin, some time after player spawn. its offline mode server and so they do a /login and then i fetch the skin from mojang and set it with the value and signature

but nothing is visible for me on client

[dyc3]

I remember this being a thing. If I recall correctly, notchian minecraft clients do some validation on skins that prevents you from overriding people's skins.